This invention relates to a block occupancy detector for model railroads in which track and equipment are miniature scale models of full sized equipment.
Model railroads have long been built with the inclusion of signal systems designed to imitate the signaling practices of their prototypes. One component of such a signal system is the block occupancy detector. The block occupancy detector is responsible for detecting the presence of any object which spans the rails within a particular section of track (a block). Many types of block occupancy detectors have been utilized over the years. Mechanical switches, electrical relays, transistors, and integrated circuits have all been used in one form or another.
Many early systems used a relay coil placed in series between the track section and the propulsion power supply which powered the locomotives (see xe2x80x9cElectrical Handbook for Model Railroadsxe2x80x9d by Paul Mallery, Simmons-Boardman, 1955). Schemes based on this method utilized the fact that the current drawn by a locomotive is substantial enough to trip the series-connected relay. Variations on this method have been created which allow less substantial loads to be detected. For example, the light bulb within a model passenger car could draw enough current to trip a more sensitive relay. These variations utilize an additional higher voltage power supply and are encumbered by the need to prevent false detection by currents which could flow back through the propulsion power supply. These relay based systems are based on direct current (DC) and only operate when a device which will conduct direct current (a DC load) is presented to the track.
When solid state devices became available, they began to replace relay circuits. Westcott""s xe2x80x9cTwin-Txe2x80x9d circuit (Model Railroader, June 1958, p. 36) is an example of such a system. Again, only a locomotive or car presenting a DC load will be detected. Variations on this method have been made with more sensitive and less expensive components as transistors became less expensive and integrated circuits became available, but the DC load limitation persisted.
With these limitations, it has been inconvenient to imitate the practice of full scale railroads in which any rolling stock can be detected by the block occupancy detector. In the practice of full sized railroads, the rails are normally insulated from each other. Any car, locomotive, hand car, or other metal obstruction which spans the rails is detectable because it forms a connection between the rails. In general, this connection is considered to be a direct connection with very low resistance. Model railroads tend to provide electrical power for locomotives and car borne accessories from the two rails. A direct connection of low resistance across these rails is thus very undesirable. For this reason, wheels on opposite sides of model cars which do not require electrical power are carefully insulated from each other. Thus, detection of the presence of the vast majority of model cars has been difficult and largely neglected.
One method for circumventing this limitation on model railroads has been to provide a highly resistive conduction path across the rails on all cars. This is accomplished either by the use of resistors connected to the wheels, or through the use of conductive paint applied so as to span the insulation between opposing wheels. This practice is generally unsightly, unreliable, and inconvenient.
Some low frequency (60 Hz) occupancy detection circuits based on alternating current have been described. The method of Small (Model Railroader, July 1947) requires that cars be equipped with resistors.
The method of Madle (Model Railroader, July 1947) utilizes high frequency alternating current but with the requirement that cars be outfitted with capacitors to bridge the rails. The sensitivity of this scheme must be limited so that the stray capacitance of the track, itself, does not cause an indication of occupancy, whereas the additional capacitance of a properly outfitted car will cause such an indication.
In 1947 Hibbs and May are mentioned as having developed a method utilizing high frequency alternating current for occupancy detection (Model Railroader, September 1947, p. 742). The circuit was said to detect a change in capacitance between the rails such as that caused by the presence of metal wheels insulated from their axles. Insufficient details are given to determine the method of operation of the circuit or its effectiveness.
Van Allen describes a scheme based on high frequency alternating current (Electronics, December 1949, p. 148, also described in Model Railroader, March 1950, p63) which overcomes some of the limitations of the scheme of Madle. In Van Allen""s method, the rails become part of a resonator which is weakly coupled to a high frequency oscillator by a resonant transformer with tunable secondary. The strength of the resonance is detected with a diode. In operation, the transformer is tuned for peak detected output with the track unoccupied. The presence of a suitably equipped car will take the resonator out of resonance and the detected output will subsequently drop. Van Allen claimed that cars which are equipped with capacitance as low as 10 picofarads can be detected with this circuit.
Richley (U.S. Pat. No. 5,752,677) describes a system for detecting minute capacitance changes by injecting a pulsating radio frequency signal onto the rails and, by the use of a balanced transformer, creating a null condition. Slight deviations from this null condition are detected as indications of occupancy. The null condition is obtained by adjustment of both capacitance and resistance in a nulling network.
Digital command control (DCC) has become an increasingly popular way to control various appliances, including locomotives, on model railroads. Among its other features, DCC allows individual locomotives to be independently controlled by encoding digital data into polarity reversals of the track voltage. Standards for DCC have been established by the National Model Railroad Association, as described in S-9.1 of that organization, and can be found at http://www.nmra.org. With this level of technological sophistication becoming commonplace in the pursuit of realistic operation, it is also desirable to provide realistic occupancy detection of unmodified rolling stock.
The object of the present invention is to provide a means for detection of unmodified rolling stock on a model railroad which is equipped with digital command control (DCC). The present invention accomplishes detection of unmodified rolling stock by measuring the slight change in capacitance of a section of track which occurs when an occupying car is present. The invention enables the capacitance of the track circuit to be measured independently of any track resistance effects. The invention is particularly well suited to systems DCC of the type wherein track voltage alternates abruptly and frequently between positive and negative extremes.
In order to accurately imitate the practice of full scale railroads while providing propulsion power through two rails, some means of detecting cars which do not present a DC load is needed. The present invention accomplishes this in the presence of DCC by the use of a current transformer, current amplifier, and correlator to form what is essentially an electrometer for the measurement of charge transferred to the empty track section with each polarity reversal of the track voltage. This charge measurement represents the capacitance of the track section, and deviations from the measurement in an unoccupied condition represent occupancy detection events.
The slight amount of current which charges the capacitance of a track section with each alternating transition of track voltage is amplified and filtered so as to create alternating pulses with amplitude proportional to the charge transferred to the track section. These alternating pulses are then passed through a correlator in which they are correlated with the alternating track voltage so as to suppress extraneous noise and to leave that component of signal which is proportional to track capacitance.
In order to greatly increase the sensitivity of the detection, some form of synchronous cancellation is provided. In the present invention, this cancellation need only to provide pulses of adjustable magnitude which can be subtracted from the track current pulses so as to allow substantial gain to follow this cancellation stage without risk of saturation of subsequent stages. The alternating cancellation pulses are then adjusted in magnitude until the output of the correlator is within the linear range of a measuring analog-to-digital converter when the track section is unoccupied. From this null condition, variations in correlator output are interpreted as occupancy events.
Such detection circuits should, for best results, be located near the track feed points, so as to minimize any overhead capacitance due to wiring. Correlator outputs from several such track sections are then connected to a common signal controller, which contains a controlling microprocessor and a multiplexed analog-to-digital converter, so that several blocks could be controlled and monitored with a common controller.
In contrast with prior art capacitive sensing systems, the present invention allows for a common rail connection, and does not require that both rails be gapped for each block. Furthermore, cancellation pulses need not be replicas of track current pulses in order to be effective. They only need to exhibit consistency in their time-integral, as presented to the correlator, for any given adjustment level. Then, only the magnitude of these pulses must be adjusted for nulling, since the time integration step inherent in the correlation process will remove any ramification of the particular shape of either the track current pulses or the cancellation pulses. There is no need for further adjustment of phase, in addition to amplitude, as with previous detection systems such as in the ""677 patent.
Variations in sensed track current pulses resulting from variations in transition time of track voltage are substantially mitigated by the integration step inherent in the correlation process. In contrast, variations in amplitude of the alternating DCC track power signal, as may occur from loading of a common booster circuit due to locomotives in other blocks, will cause variations in correlator output. However, the amplitude of this booster output is also available either at each block detection circuit, or at a common signal controller. Thus, a measurement of the concurrent track voltage magnitude can easily be made and delivered to the microprocessor by the use of an analog-to-digital converter. Compensation for changes in the correlator output due to variations in track voltage can then be made in software.